In this paper we present a novel linearly constrained (LC) RLS algorithm with variable forgetting factor (VFF), and apply to the DS-CDMA system for suddenly joined narrowband interference suppression. Under Rayleigh fading channel environment, the VFF could be employed to improve the tracking capability compared with the conventional LC-RLS algorithm with fixed value of forgetting factor (FF). From computer simulation results, we verify that the proposed scheme outperforms the conventional optimal FF LC-RLS algorithm in terms of rapid tracking capability and strong narrow-band suppression.